Currently, standardized and commercially deployed radio access technologies are proliferated. Such radio access technologies include the Global System for Mobile communications (GSM), Enhanced Data Rates for GSM Evolution (EDGE), General Packet Radio System (GPRS), wide-band code division multiple access (WCDMA), Long Term Evolution (LTE) systems, wireless local area networks (WLAN), CDMA 2000 and others.
From an end-user perspective, most of the user equipments, such as cellular phones, Personal Digital Assistants (PDA), laptop and stationary computers support multiple wireless access technologies allowing for accessing services via the “best” currently available radio access technology. Indeed, by now the notion of the “always best connected” (ABC) networks is routinely supported by wireless service operators and widely used by end-users.
Particular aspects of multi radio access technology systems in general and ABC networks in particular are access discovery and access selection. Access discovery generally assumes that as the user equipment moves along in the coverage area, it performs periodic measurements on multiple radio access technologies and frequencies for the purpose of detecting and evaluating the availability and quality of available wireless networks. For instance, a GSM/WCDMA capable mobile phone may be configured such that when it camps on GSM, it periodically scans or measures its radio environment for WCDMA cells.
Such periodic measurements, which in reality occur continuously, however, require that the user equipment operates its radio receiver de facto continuously leading to an increased battery consumption compared to the case when the user equipment keeps camping on the current radio access technology as long as there is no compelling reason to perform access discovery and selection. Such reason may be the activation of a particular service, running out of coverage of the current radio access technology, a radio link failure or other events. Also, the issue of battery consumption associated with access discovery is widely recognized.
According to one solution, the available radio access technologies of the multi-access system are associated with a priority value. In this priority scheme, user equipments do not measure on alternative radio access technologies that are of lower priority than the current one. For instance, when the user equipment camps on its highest priority radio access technology, it does not perform radio access technology detection as long as the quality of the current radio access technology is acceptable. The priority based mechanism is a scheme to minimize the inter-frequency and inter-radio access technology measurements and at same time to allow user equipments to have a preference to camp on their prioritized radio access technologies and frequencies.
A problem with continuous measurement of inter-frequency and inter-radio access technology detection/selection is the increased user equipment battery consumption. Some solutions to this problem may be to not measure for alternative radio access technologies continuously but only when some predefined conditions are met.
Some of these mechanisms apply radio access technology and/or frequency specific minimum threshold and offset values that govern when the user equipment should start measurements and should make a frequency/radio access technology selection decision. For instance, it is possible to set these values such that the user equipment never measures on GSM while camping on WCDMA. Alternatively, the user equipment can trigger measurements only when the WCDMA signal strength and signal quality drops below some threshold value.
Another category is the priority based mechanism as discussed in the previous subsection. According to such solutions, each access is assigned a certain priority value. The priority value of the current radio access technology is an important input to the user equipment measurement triggering mechanism, as discussed above.
In a more elaborate version of the priority based mechanism multiple priority lists of frequencies and radio access technologies are created. Each of these lists, consisting of an ordered list of carrier frequencies and radio access technologies, are referred to as an access pipe. Thus an access pipe is an ordered, prioritized list of radio access technologies and carrier frequencies, which radio access technologies and/or carrier frequencies may be selected by a user equipment. Within a particular access pipe, each of the radio access technologies that constitute that access pipe is associated with a distinct priority level. This distinct priority level which a given radio access technology, within an access pipe, is associated with is referred to as the class of the radio access technology.
An access pipe is associated with a class. For example, an access pipe may be of broadband, medium speed or narrow band class, which would generally be associated with user's subscription. Within an access pipe different frequencies and radio access technologies may have different priority level. For instance within access pipe A, belonging e.g. to the broadband class, LTE frequency f4, e.g. 10 MHz, may have higher priority than 3G frequency f1. These frequencies or radio access technologies within an access pipe is referred to as access pipe entities or ingredients.
The access pipe concept involves three key aspects. Firstly, some entity that has information of the overall available radio access technologies within a certain geographical area needs to construct the access pipes. For instance, the Operator may want to define three access classes such as e.g. broadband, medium speed and narrow band, each of which is associated with an access pipe. Each access pipe is an enumerated list of available accesses.
Secondly, each user equipment must learn to which access class it belongs. For this purpose various signalling and broadcasts solutions are available.
Thirdly, once the user equipment knows which access pipe it belongs to, it needs to find the list members, e.g. cells or sites belonging to the ingredients of that particular access pipe. For instance, if a user equipment belongs to access pipe “B”, and access pipe “B” comprises 3G_f3 and E-UTRAN_f5, it needs to search for 3G_f3 and, possibly, for E-UTRAN_f5.
The existing solution makes the subtle assumption for the second step above, that the user equipment is assigned the explicit list that constitutes the access pipe. This solution however has the following disadvantages and problems:
The signalling message to the user equipment is rather long and dependent of the length of the access pipe, in terms of the number of radio access technologies.
Specifically for 3GPP accesses, for user equipments in idle mode, there is no cell specific signalling available. Therefore, it is not possible to define site or location specific access pipes within a certain geographical area. For LTE, new access pipes can be signalled to the user equipment upon tracking area updates, since at those occasions non-access stratum (NAS) signalling takes place between the user equipment and the core network.
NAS is a functional layer e.g. in the UMTS protocol stack between the Core Network and user equipment. The layer supports signalling and traffic between those two elements.
It is not possible to change the definition of a particular access pipe between NAS signalling instances. For certain user equipments, this may mean that modifying the access pipe, e.g. by changing the priorities or adding and/or deleting a particular radio access technology to or from the list is not possible for a period of hours, or even days.
Further, the previously known solutions do not address a scenario in which different base station sites within a coverage area, or more specifically: within tracking area, have different capabilities leading to a situation where all possible access pipes may not be supported in all the sites.
Yet a discussed solution is to provide the details of the access pipe, to which the user equipment is assigned, to each user according to its subscription, e.g. cells with their priority belonging to access pipe A, at the time of tracking area update via NAS signalling. There are at least two problems with this later solution. Firstly this is quite detailed information depending upon the number of cells, and/or ingredients, of the access pipe. This means if provided to each user individually then the aggregate signalling load becomes quite large. Another major problem arises in homogeneous tracking areas in terms of availability of access pipes, e.g. sites have different access pipes or some sites have fewer pipes compared to others. For instance the behaviour of a user equipment associated with access pipe A would remain unspecified when it moves to a site that does not support access pipe A. The user equipment could either camp on any cell, which means that the access pipe concept would fail in this scenario. Another possibility would be that NAS signalling provide full details, i.e. super set, of all the access pipes in the entire tracking area with some default or fall back pipes. However, this would lead to large signalling overheads given the fact that this information is to be provided to each user in the entire tracking area.